I know a lot of people create custom brushes in PS, but this technique is a little different (and technically not a "brush") in that it uses the custom shape tool so brushes are resolution independent, and can scale in real time to different shapes and sizes. It is fantastic for blocking out paintings really quickly. I'll use the painting on the left of Big Wing, the main character in my animation "Little Wings", as the example. Hopefully this is helpful to you. If not, here is a video of a raccoon stealing cat food. If you do end up using this, I'd love to see what you create!
1. First I create a black painting, in this case it's a really knarly vine that I want to use as foliage. There really aren't any limits with this other than keeping it black, so feel free to go ape shit, detail is cheap. The painting is on it's own layer so it can easily be selected.
2. Select your painted layer and then the "Paths" tab. Down at the bottom of the paths tab is a little dotted line icon that kind of looks like Princess Leia. Click that to create a "work path".
3. With the work path highlighted, go to: Edit> Define Custom Shape. Give your Custom Shape a cool name and save it.
4. Type: "U" to bring up your shape tool (or you can select it in your tool palette like an old timer) and make sure you have "fill pixels" turned on in the tool settings, and "custom shape tool" selected (it looks like a wonky star). From there you can select your new custom shape from the drop down and paint away!